1. Field of the Invention
This invention relates generally to systems and methods for delivering therapeutic gas to patients, and in particular, relates to such systems and methods in which the gas delivery is tailored to the patient's breathing pattern.
2. Description of the Related Art
The application of oxygen concentrators for therapeutic use is known, and many variants of such devices exist. A particularly useful class of oxygen concentrators is designed to be portable, allowing users to move about and to travel for extended periods of time without the need to carry a supply of stored oxygen. Such portable concentrators must be small and light to be effective. Oxygen concentrators in general are implicitly limited in terms of the rate at which they can deliver oxygen to the patient, but benefit because they are only duration-limited by their access to electric power. To make the portable concentrators small and light, the rate at which oxygen is concentrated by the device is further restricted. However, use of a device called a conserver, which is placed in the product line between the concentrator and the patient, mitigates this limitation.
The conserver, many designs of which are known in the art, senses a patient's breath demand, and responds by delivering a volume of oxygen-rich gas (known as a bolus) to the patient. This bolus, which is often significantly less than the total volume of a typical inhalation, is entrained in the breath's air intake, and mixes with the air, eventually reaching the lungs, esophagus, and respiratory cavities (nose and mouth). Approximately half of an inspiration enters the lungs, where oxygen is adsorbed. Elevated oxygen concentration in this volume result in greater transfer of the gas to the blood, which enhances the health of the patient. Because the lungs can only make use of oxygen in the volume that reaches them, it is important that the bolus be delivered during the portion of an inhalation that actually reaches the lungs. As this is typically the first fifty percent of a breath, the bolus should be delivered quickly, requiring that the bolus delivery start as rapidly as possible after the start of the patient's breathe. Quick delivery of the bolus allows smaller boluses to be delivered while still satisfying the patient's need for oxygen. Thus, conservers that deliver an effective therapeutic amount of oxygen in relatively small, short bursts, constitute a more efficient use of the concentrated product gas. This allows for the design of small, lightweight concentrators that are equally effective as the large continuous flow gas supplies.
Although one of the primary motivations behind small concentrators is to allow patient freedom and mobility, the cost of these devices makes it advantageous if the concentrator is a single solution, used 24 hours a day, for all of a patient's oxygen needs. In order to be so employed, it is desirable to maximize the concentrator's efficacy while a patient is sleeping. However, there is concern in the respiratory care field that conserver-based delivery of oxygen is not as effective as continuous flow gas supplies at maintaining patient blood oxygen saturation levels during sleep.
One cause for this nighttime desaturation concern surrounds the conserver's sensitivity, or the inhalation vacuum pressure (typically sensed through a nasal cannula) that results in a bolus delivery. In order to reduce false triggers (bolus delivery when no breath has occurred), breath detection, which is accomplished by measuring inhalation vacuum pressure typically through a nasal cannula, is set to a level corresponding to normal daytime breathing and activity patterns. If the pressure at which the conserver triggers a bolus is too low, normal activity may cause false firing, which can be disconcerting to patients and is ineffective oxygen therapy as much of this oxygen does not reach the lungs. However if the trigger pressure is too high, the conserver does not recognize a breath until a significant portion of it has already been inspired, which reduces the efficacy of the delivered bolus. Thus, it is desirable to have the conserver's breath sensitivity be as high as possible such that bolus delivery speed is accelerated, so long as this sensitivity remains low enough to avoid activity-induced false firing.
In addition, conserving devices typically deliver a pre-determined volume of gas in response to patient breath demand. During sleep, the normal daytime trigger levels may be too high, and the associated bolus volumes may not be adequate to maintain required blood oxygen saturation levels. Moreover, during sleep, some patients are shallow and/or irregular breathers, such that nighttime breathing for these patients may not generate enough vacuum pressure to trigger bolus delivery. In some cases, due to irregular breathing patterns, the conserver may not detect every breath, resulting in breath skipping. In either of these cases, a bolus may not be triggered often enough to deliver enough oxygen to the patient over time. Many conservers are equipped with a breath detection or apnea alarm that notifies the user when no breath has been detected for some period. However, the alarm can awaken the sleeping patient, which makes use of the conserver not feasible.
Since most therapeutic gas systems deliver gas and sense patient breathing through a nasal cannula, patients who breathe through their mouths at night may never trigger bolus delivery. It is known within the respiratory care field that while patients are breathing through their mouths, they are entraining oxygen rich gas stored in the nasal passages with each inhalation. As such, large air supply systems simply deliver continuous flow to the nasal passages. However, continuous flow oxygen delivery, when not inhaled through the nose, may result in a cloud of oxygen-enriched gas around the face with oral inhalation entraining only some of this gas. As a result, the rate of oxygen delivery in these continuous flow oxygen systems often have to be increased during sleep to compensate for these inefficiencies in delivery.
Thus, it is apparent that new approaches to conserver-based delivery for sleep mode operation are desired in order to provide patients with the opportunity to use the small air concentrators 24 hours a day.